Natural circulation calandrias

CALANDRIAS. Vertical shell-and-tube units, known as calandrias, natural-circulation or thermosiphon reboilers, are generally the tnost economical vaporizers for distillation and evaporation operations. A typical arrangement is shown in Fig. 

Fig. 1. Natural circulation evaporators where C = condensate, E = entrainment return, F = feed, N = noncondensibles vent, P = product or concentrate, S = steam, V = vapor, and M = knitmesh separator (a) horizontal-tube, (b) short-tube vertical, (c) propeller calandria, and (d) long-tube recirculating.

No detailed tests have been reported for the performance of propeller calandrias. Not enough is known regarding the performance of the propellers themselves under the cavitating conditions usually encountered to permit predicting circulation rates. In many cases, it appears that the propeller does no good in accelerating heat transfer over the transfer tor natural circulation (Fig. 11-23). 

Two general types of evaporators are used, and their names refer to the type of circulation used to transfer heat to the liquor for evaporating the water. Natural circulation evaporators rely on a thermosiphon to circulate liquors while forced circulation units use a pump to achieve the required circulation. The heating tubes may be inside or outside the evaporator body, but most designs, especially the older calandria style evaporators, use internal tubes for heating (Figure 2). 

Natural circulation in the standard short tube evaporator depends upon boiling. Should boiling stop, any solids suspended in the liquid phase will settle out. The earliest type of evaporator that could be called a forced-circulation device is the propeller calandria illustrated in Fig. %(e). Basically a standard evaporator with a propeller added in the downcomer, the propeller calandria often achieves higher heat transfer rates. The propeller is usually placed as low as possible to avoid cavitation and is placed in an extension of the downcomer. The propeller can be driven from above or below. Improvements in propeller design have permitted longer tubes to be incorporated in the evaporator. 

Control of natural circulation calandrias presents some problems not found in other heating elements. When heating with condensing vapors. 

What is the temperature of the top head for natural circulation calandrias A temperature higher than the liquid temperature may indicate inadequate circulation for some reason ... 

Heat flux in natural-circulation calandrias. (4fter frank and Prickett. )... 

A steam-heated natural-circulation calandria is to be designed to boil 5000 kg/h of chlorobenzene at atmospheric pressure, (a) Approximately how much heat-transfer surface will be required (h) How much area would be required if the average pressure in the calandria were 0.5 atm abs The normal boiling point of chlorobenzene is 132.0 C its critical temperature is 359,2°C. 

A vapor-recompression evaporator is to concentrate a very dilute aqueous solution. The feed rate is to be 30,000 Ib/h the evaporation rate will be 20,000 Ib/h. The evaporator will operate at atmospheric pressure, with the vapor mechanically compressed as shown in Fig. 16.12 except that a natural-circulation calandria will be used. If steam costs 8 per 1000 lb, electricity costs 3 cents per kilowatthour, and heat-transfer surface in the heater costs 70 per square foot, calculate the optimum pressure to which the vapor should be compressed. The overall compressor efficiency is 72 percent. Assume all other costs are independent of the pressure of the compressed vapor. To how many effects will this evaporator be equivalent ... 

Used solvent is liable to foul heat exchanger surfaces and so will almost always be on the tube side of a shell and tube heat exchanger with steam on the shell side. While it is possible to use a natural circulation external calandria if the solvent to be evaporated is clean, forced circulation is more reliable if the solvent contains residue (Fig. 4.1) despite the fact that it may be a diliicult duty as regards both cavitation and seal maintenance. 

Natural circulation calandrias (or thermosiphon reboilers) depend upon density differences to produce required flow rates. Vaporization creates an aerated liquid with a density less than that of the liquid in the system. The hydraulic head resulting from this density difference causes the fluid in the system to circulate. Circulation rates are high with liquid-to-vapor ratios ranging from 1 to 50. 

Figure 10-1 Heat transfer versus common liquid level-natural circulation calandria.

Figure 10-2 Natural circulation calandria-mechanlsm of operation-sensible heat zone and boiling zone.

Figure 10-3 Natural circulation calandria-mechanism of operation-mean temperature differences.

Figure 10-5 Piping arrangement-parallel natural circulation calandria.

Flashing feed is often introduced at the inlet of the calandria. The effects of flashing must be evaluated when establishing natural circulation flow rates. It may be advantageous to introduce flashing feed in the vapor body. 

Liquid level in the vapor body is an important variable affecting operation of natural circulation calandrias. Normally units are operated with the evaporator liquid level at the top tubesheet of the calandria. For non-fouling fluids, the liquid level can be lowered to the optimum value in order to minimize heat transfer surface or maximize performance. The optimum value is approximately half the distance between the top and bottom tubesheets of the calandria, and will vary with each system. The liquid level should not be appreciably above the top tubesheet and certainly should not be maintained above the caiandria outlet nozzle. Liquid levels above the vapor return will limit the performance of the calandria and may result in damage to the evaporator. Flow instabilities may also be experienced. 

Axial flow units perform well as natural circulation calandrias. Spiral-plate units are also effective for condensers and for heat recovery applications. 

What type of calandria will be used forced or natural circulation ... 

Figure VIII-1 shows a simplified schematic diagram of the nuclear steam supply system with the Package-Reactor. The concept resembles a calandria-type pressurized heavy water reactor (e.g., the FUGEN advanced thermal reactor (ATR) or CANDU reactors) since all these employ pressure tubes. But the Package-Reactor is somewhat different from the ATR or the CANDU. The Package-Reactor employs natural circulation with two-phase flow for core cooling and has no recirculation pumps. The height of the pressure tubes of the cassettes is required to be as low as possible to attain a compact unit. Two-phase flow with high void fractions similar to BWRs is adopted to attain natural circulation with a cassette height of 6 m and a fuel rod length of 3.65 m.

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